The present invention relates to a method for filling foam-structure or fiber-structure electrode plates with pastes of active substance for galvanic cells, particularly for electrical accumulators, as well as to a composition of the pastes of active substance.
Electrode plates having a foam or fiber structure are being increasingly widely used because they present advantages compared to the cheap cast and expanded metal grids as regards electrical load capacity and useful life. The fibers of these plates may consist entirely of metal, such as nickel-plated steel wool for example, or they may be formed by metallization of plastic non-woven fabrics, according to U.S. Pat. No. 3,560,262 for example.
Foam-structure plates, which are also frequently known as reticulated plates, can be produced by the metallization of open-cell foam materials such as carbonized foam materials according to Great Britain patent specification No. 1,211,428 for example, or of foam materials according to German Published, Unexamined Patent Application (DE-OS) 2,427,422 or by foaming a plastic material containing a metal powder and subsequent pyrolysis according to European Patent Specification (EP-PS) 87,160. For reasons of production, the pores of these reticulated plates form polyhedra of approximately spherical shape and vary far less in shape and size than the pores of plates which consist of non-woven textile materials. Due to the gas bubble sizes dictated by the process technology, the pore diameter of the reticulated plates is much greater than that of sintered plates produced by powder metallurgy, and, a considerable reduction in the thickness of the reticulated plates is generally undertaken by rolling or pressing. The plastic fraction is almost always removed from the reticulated plates by pyrolysis, so that all-metal plates with no plastic fraction are obtained in the final result.
The following classification of electrode plates categories by pore sizes and pore shapes can be made:
a) Very large pores in the millimeter range in the form of bi-dimensional holes regularly arranged: Cast grids, expanded metal grids, nets.
b) Large pores, polyhedra approximating to spherical shape, pores cross-linked three-dimensionally, narrow pore-size distribution: Reticulated (foam) plates.
c) Medium-sized pores, anisodimensional (widely different dimensions of a pore in different physical directions), pore delimitation by partially elongate bars, three-dimensional cross-linkage of the pores: fiber plates and compressed foam materials.
d) Small pores, three-dimensionally cross-linked, pore delimitation by sintered necks, approximately 60% of the total pore volume in the form of pores in the range 4 to 15 micrometers (Falk and Salkind, Alkaline Storage Batteries, Publisher J. Wiley 1969, Page 122): powder metallurgical sintered plates.
Various techniques which are oriented by the above enumerated pore characteristics, have been developed for the introduction of the active substance by filling or impregnation.
The filling of the plates mentioned under category a), which are provided with two-dimensional, optically transparent hole patterns, is generally done by pressing in a paste of active substance mechanically, as for the electrodes of lead accumulators, where this technology has been applied to cast grids and expanded metal grids. Rollers or doctor blades are used as pressure-imparting elements. Since the active substance must be sufficiently firm not to flow out of the holes after pasting, but sufficiently plastic to be still spreadable, high viscosity thixotropic pastes are used (e.g. German patent specification Nos. (DE-PS) 2,517,368 or (DE-PS) 2,602,904) which become sufficiently fluid under the pressure of the roller or of the doctor blade.
For narrow-pored plates with a three-dimensional pore network, this method becomes more difficult the narrower the pores are. Only chemical or electrochemical precipitation methods of embedding the active substances have therefore been successful for sintered plates, because the solutions can penetrate even into very small pores. However, the methods which operate with solutions suffer from disadvantages of tediousness, accompanying frame corrosion and also contamination of the precipitation products by anions of the initial compound, nitrate, sulphate or chloride ions being typical for example. The tediousness of the filling is due, in principle, to the much lower ion concentration in the solutions as compared to the solids, so that much time is required to build up the desired high solid concentration of the active substance in the pore volume of the electrode.
Both the precipitation impregnations known for category d) and also mechanical filling methods with suspensions and pastes have been described for the foam-structure and fiber-structure plates mentioned under categories b) and c). Naturally, the less expensive mechanical methods encounter increasing difficulty of realization with a reduction in the pore size, because suspensions and pastes do not penetrate into a network of fine pores as easily as solutions.
A filling with a paste of active substance with agitation is described for reticulated plates with large pores between 230 and 2,540 micrometers in German patent specification No. (DE-PS) 1,596,023 (which correspond to U.S. Pat. No. 3,287,164). The liquid is 30% caustic soda solution. No information is given as to the viscosity of the paste, the agitation conditions (frequency, intensity, arrangement). However, the filling of such large pores presents no problems.
The filling of a foam plate with a pore diameter of 100 to 500 micrometers without metallization is described with similar terseness in German Published, Examined Patent Application (DE-AS) 1,108,759. According to this document an active substance placed in suspension is pressed in, optionally with agitation. However, according to the description the active substance suspension cannot be a paste.
An apparatus for the impregnation of reticulated plates (foam metal) with paste, likewise with no disclosure concerning the details of the pore size, is described in U.S. Pat. No. 4,217,939. The plate is guided horizontally on a perforated plate over a paste tank, in which the paste is moved by stirrers and pressed upwards into and round the plate, the paste being swept into the plate from above by reciprocating doctor blades. However, it is not immediately possible then to displace the air out of the pores of the plates by the paste. On the contrary, a preimpregnation of the plate with water is necessary for this purpose. The dilution of the paste with this water creates difficulties in adjusting the concentration of the paste.
The filling of fiber-plate electrodes with paste is described in a plurality of applications.
U.S. Pat. No. 3,262,815, corresponding to Great Britain patent specifications No. 1,109,524, describes three different methods for introducing a suspension of an active substance into a fiber plate: 1) working in mechanically, 2) depth filtration and 3) introducing the plate into a mechanically moved (agitated) bath, where the plate or the bath may be set in motion. None of these three methods operates satisfactorily by itself, because the disclosure is directed at a process which should combine all three methods. Thus, after treatment in accordance with method step 3) noted above, the plate should still undergo an after-treatment according to method steps 2) and 1). No information is given as to the mechanical characteristics of the paste; however, the treatment according to method step 3) alone clearly does not provide a satisfactory filling.
Then again, in a later application, German Published, Unexamined Patent Application (DE-OS) 2,436,704, the same applicant refers to the basic/process of working in a thixotrophic paste by pressure roller application, as already described above for lead grids, except that the method is applied to nickel fiber plates of 92% porosity and 1.7 mm thickness (Example 1). After filling, these plates are compressed to 0.9 mm thickness. This high compression, which would reduce the porosity of an unfilled plate from 92% to 85%, indicates that a paste filling into the finished frame of final thickness clearly does not succeed, a double paste application being required even for the still uncompressed frame of 92% porosity.
German Published, Unexamined Patent Applications (DE-OS) 2,427,421 (which corresponds to U.S. Pat. No. 3,877,987) and (DE-OS) 2,427,422 (which corresponds to U.S. Pat. No. 3,926,671) refer in identical texts to the possibility of filling fiber plates with suspensions of active substance which are pourable and therefore highly fluid. The suspension is poured onto the horizontally positioned plate and a vibrator electrode arranged parallel to the frame, which is in communication with an ultrasonic generator, and in combination with vacuum ensures a substantial filling, which however still requires completion by a following precipitation impregnation. According to the descriptions in these applications, the suspensions of active substance are not pastes; nor is the term paste used.
The specifications discussed above refer to plates which must be classified under a specific category a) to c) of the above-mentioned frame classification according to the pore size. However, a vibration filling with pastes under the influence of ultrasonic vibrations, and optionally of vacuum, is disclosed by German Published, Examined Patent Application (DE-AS) 1,287,663, which is claimed to be useful both for fiber plates of category (c) and also for sintered plates of category (d) produced by powder metallurgy. By this filling method the paste is applied in a thin uniform layer onto the ultrasonic vibrator, and a pressure element presses the paste layer together with the plates strongly against the vibrator, a perforated foil being placed between electrode plate and pressure element. No detailed information is given as to the flow characteristics of the paste, except for the example, according to which the paste should have a consistency somewhat like spreadable butter.
The horizontal positioning of the plate, which is adopted for all filling processes under the influence of vibrations, is considered unavoidable because in the vibration field the dispersed solid has a tendency to settle downwards, whereas the liquid moves upwards (See German Published, Examined Patent Application (DE-AS) 1,287,663). However, considerable disadvantages are associated with the horizontal positioning. It is known for instance for the filled plate to stick very firmly to the pressure plate or to the surface of the vibrator, which has the result that special removal technologies were proposed (See German Patent Specifications (DE-PS) 1,210,417 for example) or that the corresponding parts threatened by sticking are provided with an antiadhesion coating or with an antiadhesion foil. Furthermore, the production process according to the two last mentioned patents, whereby the plate to be filled is sandwiched horizontally between two layers of the active substance and is then filled under pressure and the influence of vibrations, is naturally also extraordinarily complicated.
To sum up, it can be stated that in the use of pourable suspensions, such as they are described in German Published, Examined Patent Application (DE-AS) 1,108,759, German Published, Unexamined Patent Application (DE-OS) 2,427,421, U.S. Pat. No. 3,262,815, U.S. Pat. No. 3,877,987 or U.S. Pat. No. 3, 926,671 for example, a single filling operation does not effect an adequate filling of the pores, so that a plurality of impregnation passes or following impregnations are recommended.
In the use of pastes of high solids content, i.e. pastes being understood to mean, according to Rompp, Chemielexikon 8th Edition 1985, Volume 4, Page 3006, solid/liquid dispersions of doughy consistence, and therefore not pourable suspensions, the difficulties known from U.S. Pat. No. 4,217,939 and German Published, Examined Patent Application (DE-AS) 1,287,663 as described above result.
An object of the present invention is to provide a method for the filling of foam-structure or fiber-structure electrode plates with a paste of active substance which is simple, fast and can be performed without a major outlay of apparatus or effort.
A further object of the present invention is to provide a paste of active substance suited for the method of filling the electrode plates of the present invention.
Thus, in accordance with preferred embodiments of the present invention, the electrode plates to be filled are immersed in a tank filled with a paste of active substance, in which the active substance is set in vibration. The paste in the tank has an active substance content of about 25 to 60 vol %, with a maximum granulometry of the active particles of about 0.04 mm and a yield value of about 10 to 120 Pa at about 20.degree. C. and a plastic viscosity of about 0.1 to 1 Pas at about 20.degree. C. The viscosity particulars here refer to a paste which is not in vibration. This is because a reliable viscosimetry is not known for a vibrating paste.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.